The invention concerns in general ceramic bodies. In particular, the invention concerns such ceramic bodies on the basis of aluminum titanate.
Such ceramic bodies are used, for example, as filter devices for soot particle removal, hot gas filtration, liquid filtration as well as in exhaust gas after treatment of spontaneous ignition internal combustion engines, in particular in diesel engine-operated as well as gasoline engine-operated motor vehicles. Moreover, the aforementioned ceramic bodies can also be used as heat insulation material. Usually, such filters are made of a ceramic material, for example, cordierite or silicon carbide. Ceramic bodies of cordierite find use in a plurality of high temperature applications, for example, catalytic converters, NOx adsorbers, electrically heated catalysts, chemical process substrates, and also diesel fuel particulate filters.
In the filtration of diesel exhaust gases, cordierite as an inexpensive material that has a low thermal expansion coefficient has been the material of choice for a long time. Porous cordierite ceramic filters have been used in the form of wall-flow filters since the early '80s for removal of particles in the exhaust gas flow of diesel engines. Wall-flow filters have in general a cylindrical shape with two end faces and a jacket surface and have a plurality of flow passages for the exhaust gases of the diesel engine, the flow passages passing through the filters from the first end face to the second end face and extending substantially parallel to the cylinder axis. The cross-sectional shape of the wall-flow filters depends on the mounting conditions at the vehicle. Widely used are filter bodies with round, elliptical or triangular cross-section. The flow passages have usually a square or hexagonal cross-section and are arranged in a tight pattern across the entire cross-section of the filter body.
A diesel particle filter combines ideally a low thermal expansion coefficient, a minimal pressure loss, high-strength, and minimal cost. Problems that may be encountered upon use of cordierite comprise low volumetric thermal capacity and low thermal conductivity; this leads to unacceptably high temperatures or temperature peaks during operation as well as a minimal thermal and chemical resistance. Moreover, inorganic particles contained in the diesel exhaust gas may react with the cordierite and may cause filter failure.
An alternative material to cordierite for producing diesel particulate filters is silicon carbide (SiC). Even though this material has a high-volumetric thermal capacity as well as a high thermal conductivity, it also has a low resistance to temperature changes as a result of a relatively high thermal expansion and a high modulus of elasticity. This makes it necessary to segment SiC filters in order to prevent failure during use. Also, the processing requirements (i.e., high temperatures, inert atmosphere, and segmentation) result in high manufacturing costs.
Recently, ceramic filter elements on the basis of aluminum titanate have been developed that exhibit suitable properties for use at high temperatures (e.g. hot gas filtration) such as, for example, in vehicle exhaust gas control and diesel exhaust after-treatment systems, e.g. as diesel particulate filters or also particulate filters for gasoline engines. Aluminum titanate is the stoichiometric mixed phase of aluminum oxide and titanium dioxide. It is characterized by a low thermal conductivity, a low modulus of elasticity as well as a macroscopically minimal thermal expansion coefficient and an excellent resistance to temperature changes correlated therewith.
DE 602 17 084 T2 discloses a honeycomb structure with a plurality of honeycomb segments which are connected to a unitary body. The main component of each of the honeycomb segments comprises at least one of silicon carbide, silicon nitride, cordierite, aluminum oxide, mullite, zirconium dioxide, zirconium phosphate, aluminum titanate, titanium dioxide, and combinations thereof.
DE 10 2006 040 739 A1 discloses a filter for removal of particles from a gas stream, in particular of soot particles from an exhaust gas stream of an internal combustion engine, comprising a filter body of a ceramic filter substrate wherein the filter substrate is coated with a porous protective layer of a coating material. The coating material is selected from the group consisting of aluminum oxides, aluminum hydroxide, titanium dioxide, silicon dioxide, zirconium dioxide, cerium oxide, aluminum silicate, magnesium aluminum silicate, cordierite, mullite, silicon carbide, aluminum titanate, zeolite, quartz, glasses, mixtures thereof, and mixed oxides thereof.
Finally, WO 2005/046840 discloses a ceramic body for use as a diesel particulate filter with a composition that comprises: a(Al2O3.TiO2)+b(CaO.Al2O3.2SiO2)+c(SrO.Al2O3.2SiO2)+d(BaO.Al2O3.2SiO2)+e(3Al2O3.2SiO2)+f(Al2O3)+g(SiO2)+h(Fe2O3.2TiO2)+i(MgO+2TiO2) wherein a, b, c, d, e, f, g, h, and i are weight fractions of each component so that (a+b+c+d+e+f+g+h+i)=1 and the following conditions are fulfilled: 0.5<a<0.95; 0≦b≦0.5; 0≦c≦0.5; 0≦d≦0.5; 0≦e≦0.5; 0≦f≦0.5; 0≦g≦0.1; 0≦h≦0.3 0≦i≦0.3; b+d>0.1. This is a mixture of aluminum titanate and a glass phase wherein the glasses are those of alkaline earth, alkali, silicon dioxide, aluminum dioxide, alkali and alkaline earth glasses. Moreover, the mixture contains a mineral phase, for example barium, calcium, and strontium feldspars. The ceramic bodies are produced by plastic shaping methods, for example, extrusion. A disadvantage is that feldspar is a natural raw material that is subject to fluctuations and also may contain impurities. Strontium feldspar is moreover very difficult to procure. Further disadvantages of the barium, calcium, and strontium feldspars are their high sintering temperature and difficult processibility. Calcium and in particular barium compounds are irritant; strontium components are weakly radioactive.
In addition to the manufacture by plastic shaping methods, e.g. extrusion, such ceramic bodies or filter devices are also produced in that a non-flammable support web is impregnated with a ceramic slip and subsequently a rigid filter body is formed. In Patent Abstracts of Japan JP 63134020 A, for example, a ceramic filter element for an exhaust gas filter in an internal combustion engine is disclosed that is comprised of a corrugated, spirally wound filter web. For producing the filter web, heat-resistant inorganic fibers are mixed in an aqueous suspension with ceramic powder and processed to a web. Several stacked webs are then rolled to the desired shape of the filter body wherein between neighboring filter webs honeycomb-shaped flow passages are formed. The filter body is subsequently fired at high temperature.
WO 2006/005668 discloses a further method for producing a ceramic filter element in an exhaust gas filter for internal combustion engines. In this method, first a flammable, non-ceramic support web is impregnated with a ceramic slip and subsequently, in the desired geometric shape, fired to such an extent that the support web is combusted and a rigid filter body remains.
The invention has the object to provide ceramic bodies on the basis of aluminum titanate that have a high thermal and chemical resistance, high long-term stability/resistance to breakdown in reducing and oxidizing atmosphere as well as excellent material characteristics (in particular, thermal expansion coefficients and, correlated therewith, very good resistance to temperature changes).